Display screen or portion thereof with a graphical user interface



The FIGURE is a front view of a display screen or portion thereof with a graphical user interface showing the new design.

The outermost broken line rectangle illustrates a display screen or portion thereof and forms no part of the claimed design. The remaining long dash-long dash and short dash-short dash broken lines illustrate portions of the graphical user interface that form no part of the claimed design. 

CLAIM The ornamental design for a display screen or portion thereof with a graphical user interface, as shown and described. 